var b1 = true;//字面量
var b2 = new Boolean(1);//构造函数
var b3 = Boolean(1);
//console.log(b2, typeof b2);
//console.log(b3);
//false: 0  undefined  null  ''  NaN
//[]  {}

//console.log( Boolean({}) );

// !!隐式转换为布尔型
console.log( !!null ); //false